A high-frequency power amplifier, for example, a microwave power amplifier, includes active devices, such as an electric field effect type transistor, passive components, such as a resistor and a capacitor, and circuit elements, such as a microstripline which transmits a high frequency signal. The circuit element of such a microwave power amplifier is mounted on a high frequency package as exemplified in U.S. Pat. No. 5,692,298 and JP, P2000-510287A.
As for the high frequency package shown in U.S. Pat. No. 5,692,298 and JP, P2000-510287A, a resonance frequency of a seal ring is determined by the length of the seal ring. If the resonance frequency becomes equal to the working frequency of the package, isolation between an input terminal and an output terminal which are provided under the seal ring will be affected.
JP, P2001-196500A and JP, P2002-289720A discloses the structure of a high frequency package for avoiding resonance of a seal ring in a working frequency.
In JP, P2001-196500A, the seal ring on a side wall is grounded via penetration holes. The resonance can be avoided by providing many through holes, however, this is costly.
In JP, P2002-289720A, to avoid the resonance, a metalized layer is formed all over the rear face of a cap. Resonance can be avoided by making the cap with metal or forming a metalized layer on the rear face of the cap, however, the frequency characteristic of a wire inside a cavity is problematic.